An ink jet recording apparatus has been known, which has a recording head for discharging ink as droplets onto recording paper to print an image. The ink jet recording apparatus is provided with at least an ink container containing ink, to supply the ink to the recording head. The recording head is provided with nozzles and oscillation blades. The oscillation blades are driven individually by piezoelectric elements to change pressure inside the associated nozzles, so that the ink is sucked into and discharged from some of the nozzles when the associated ones of the oscillation blades are driven.
Because the ink is a consumable material, the ink container is often formed as a cartridge that is removably attached to the ink jet recording apparatus, so as to make it easy to supplement the ink jet recording apparatus with the ink. In order to notify the user of necessity to replace the cartridge type ink container, hereinafter called the ink cartridge, the ink jet recording apparatus using the ink cartridge is often provided with an ink run-out detector for checking if the ink in the ink cartridge is used up.
For example, Japanese Laid-open Patent Application No. 2002-86756 discloses an ink jet recording apparatus, wherein an ink holding member, which swells as it absorbs the ink, is affixed to inside an ink cartridge, and an optical sensor detects shrinkage of the ink holding member with reduction of the ink, so as to detect the ink run-out. According to an ink jet recording apparatus disclosed in Japanese Laid-open Patent Application No. 2004-188630, a detection paper strip is attached to an ink cartridge, such that the detection paper strip is colored with ink by an area proportional to the consumed amount of the ink. When an optical sensor detects that the colored area extends to a predetermined position, it is determined that the ink is used up.
However, according to the detection method disclosed in the first prior art, it is difficult to detect the ink run-out with accuracy, because the degree of shrinkage of the ink holding member varies depending upon wavy movement of the ink in the ink cartridge and environmental conditions like humidity. Also the detection method of the second prior art does not ensure accurate detection of the ink run-out, because the extension of the colored area is not always proportional to the consumed amount of the ink, but affected by blotting of the ink on the detection paper strip. Furthermore, both of these prior arts use the optical sensor for detecting the ink run-out based on the intensity of light received on a light receptive portion of the optical sensor. Therefore, when the ink cartridge is not attached to the ink jet recording apparatus, the optical sensor will confuse the absence of the ink cartridge with the ink run-out.